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If you collected all of Earth's water into a sphere, how big would it be?

Imagine the Earth in your mind's eye. Now round up all the water on the planet into a sphere (we're talking oceans, icecaps, atmosphere, everything — even the water bound up in you and me). How big do you think that sphere would be compared to the Earth?

Got your answer? Our water sphere would have a diameter of 1,385 kilometers (about 860 miles), and span the distance from Salt Lake City, Utah to Topeka, Kansas. A sphere this far across would have a volume equal to about 1,386 million cubic kilometers (roughly 332,500,000 cubic miles). Those might sound like big numbers (and they certainly are big numbers) but wait until you see this sphere beside the Earth.

The artists' conception shown here [click here for hi-res] will help you visualize the size of our ball of water relative to rest of the planet. By comparison, the Earth measures a staggering 12,256 km in diameter, dwarfing the little blue sphere — a "little blue sphere" that contains more than enough water to cover over 70 percent of our planet's surface, and fill every life form on Earth with H2O molecules. (Those looking for a similar size comparison at home can use a basketball to represent the dry Earth, and a nickel to illustrate the diameter of our water sphere.)

Pretty incredible, right? My initial estimate was way off. (I overshot. By a lot.) It just goes to show how when you're dealing with areas and volumes on planetary orders of magnitude, it's surprisingly easy to lose your sense of scale. [USGS via Philip Yam]